This invention relates to an axial air gap-type electric motor, and more particularly to the structure of permanent magnets and rotor cores provided at a rotor.
A conventional axial air gap-type electric motor has, for example, a structure shown in FIG. 8.
FIG. 8 is a cross-sectional view showing the overall construction of the basic axial air gap-type electric motor 90, and FIG. 9 shows a stator 92 of the axial air gap-type electric motor 90, and FIG. 9A is a perspective view of a stator core, and FIG. 9B is a front-elevational view of the stator core. FIG. 10 shows a rotor, and FIG. 10A is a perspective view of the rotor, and FIG. 10B is a side-elevational view of the rotor.
In the axial air gap-type electric motor 90, the rotor 94 is fixedly mounted on a rotation shaft 98 rotatably mounted on a casing 96, as shown in FIG. 8. As shown in FIG. 10, the rotor 94 has a cylindrical shape, and a plurality of permanent magnets 80 are provided in the rotor 94, and are arranged in a circumferential direction. The permanent magnets 80 are completely embedded in the rotor 94. The permanent magnets 80 are arranged to alternate in polarity (South and North poles) at each side of the rotor 94, that is, in a common plane.
On the other hand, the annular stators 92 are disposed within the casing 96, with the disk-shaped rotor 94 sandwiched therebetween. As shown in FIG. 9A, the stator 92 includes the stator core 92a as a main component, and a plurality of teeth 92a1 are formed on and project from that side (or face) of the stator 92 opposed to the rotor 94. A conductor (wire) is wound around each tooth 92a1 to form a coil 92b as shown in FIG. 9B, and when an electric current is cause to flow through each coil 92b, a magnetic pole is formed at the tooth 92a1. Namely, the stator 92 is formed by the stator core 92a and the coils 92b. Thus, in the axial air gap-type electric motor 90, the magnetic poles of the stator 92 are opposed to the permanent magnets 80 of the rotor 94 in a direction parallel to the rotation shaft 98.
When an electric current is caused to flow sequentially through the coils 92b, the teeth 92a1 are sequentially magnetized to form a rotating magnetic field. The permanent magnets 80 of the rotor 94 interact with the rotating magnetic field, and therefore attractive and repulsive effects develop to thereby rotate the rotor 94, so that a magnet torque can be obtained.
In the axial air gap-type electric motor 90, in order to obtain a reluctance torque in addition to the magnet torque developing because of the presence of the permanent magnets 80, magnetic components (ferromagnetic components) are provided respectively in predetermined portions of the rotor 94. These magnetic components are so arranged that they can be attracted by the magnetic poles sequentially formed at the stator 92 so as to rotate the rotor 94. More specifically, there are provided the eight magnetic components 84 each disposed between the corresponding two adjacent permanent magnets 80 so as to be attracted by the magnetic poles of the stator 92. The permanent magnets 80 are embedded in a rotor core 81.
As described above, the attractive and repulsive effects are produced by the interaction between the rotating magnetic field (produced at the stator 92) and the permanent magnets 80 of the rotor 94, so that the magnet torque develops in the rotor 94. Simultaneously with this operation, the magnetic components 84 are attracted by the teeth 92a1 magnetized by the rotating magnetic field produced at the stator 92, so that a reluctance torque is produced. Namely, in the axial air gap-type electric motor 90 provided with the above magnetic components 84, the sum of the magnet toque and the reluctance torque can be used as the motor torque.
Incidentally, the direction of the magnetic flux in the axial air gap-type electric motor 90 is parallel to the rotation shaft 98, and in the case where the magnetic component 84 is disposed between any two adjacent permanent magnets 80 as described above, an eddy current is produced in the magnetic component 84. The generation of such eddy current is the cause of an energy loss such as heating. Therefore, in order to suppress the generation of eddy currents, a dust core material is used to form the rotor 94, and by doing so, an electric resistance of each magnetic component 84 in a plane perpendicular to the magnetic flux is set to a high value (see, for example, Patent Literature 1).
However, the magnetic force faces of the permanent magnets of the rotor are opposed to the teeth faces of the stator, and therefore when such an electric motor is used for a long period of time or when a magnetic force of above a predetermined level is produced by the stator, there has been encountered a problem that a so-called demagnetization phenomenon, in which the magnetic forces of the permanent magnets are lowered, occurs, thus lowering the ability of the electric motor.
[Patent Literature 1]
JP-A-2005-94955 (Pages 5 to 6, FIG. 6)